JP2004-346830A, published by the Japan Patent Office in 2004, discloses air-fuel ratio control of an internal combustion engine for a vehicle in relation to an operation of a brake booster. The brake booster is connected to an intake passage downstream of a throttle of the internal combustion engine. The brake booster assists a braking pressure supplied from a master cylinder to brake cylinders by utilizing a difference between an intake negative pressure in the intake passage and atmospheric pressure in response to depression of a brake pedal of the vehicle.
When the brake pedal is released from depression, air in the brake booster is recirculated into the intake passage. The prior art device regulates the air pressure in the brake booster when the engine is started to be equal to the atmospheric pressure, thereby regulating the amount of air recirculated from the brake booster to the internal combustion engine to a constant level. By thus regulating the amount of air recirculated from the brake booster to the internal combustion engine to a constant level, the air-fuel ratio of an air-fuel mixture supplied to the internal combustion engine can be controlled to a constant ratio.